Sexual transmission is the major cause of HIV infection in women. Both the World Health Organization and the National Institutes of Health have identified an urgent need for understanding heterosexual HIV transmission and the relevant physiology of the female reproductive tract (FRT) to support HIV prevention efforts. Because recent efforts to enhance mucosal defenses against STIs using topical microbicides have failed, a better understanding of FRT immunity is warranted. Despite knowledge of how FRT physiology changes in response to penile-vaginal intercourse (PVI), there is little understanding of how PVI affects HIV-relevant FRT immunity. Proposed experiments test effects of PVI on FRT immune mediators relevant to HIV infection. Given the acknowledged influence of sex hormones on HIV infection rates, these experiments also detail how PVI changes FRT immune mediators and sex steroids in parallel. The interdisciplinary research team is comprised of biomedical, sexual health, and HIV immune scientists and physicians. The proposed research includes three specific aims: (1) define effects of PVI on protein mediators of innate and adaptive immunity at the vaginal mucosa and systemic sex hormone concentrations; (2) determine effects of PVI on the capacity for vaginal fluids to influence the vulnerability of lymphocytes and macrophages to HIV infection in vitro; (3) characterize the influence of PVI on antimicrobial peptide (AMP) concentrations in vaginal fluid. Within-subject data will be obtained from 100 premenopausal adult women in long-term heterosexual relationships. Women will engage in PVI as the experimental condition, and four activities controlling for important elements of PVI (exercise, cuddling, self-stimulation, and quiet time) in their own homes, self-collecting FRT mucosal and salivary samples and completing self-report measures before, 15 minutes after, and the morning following each activity at times selected because of relevancy to vaginal microbicide applications. This repeated measures paradigm will examine whether FRT immunity is specifically modulated by PVI compared to four control activities of a social nature. FRT specimens will be used to quantify protein and lipid mediators, which reflect the state of mucosal immune defenses; salivary samples will be used to measure sex steroids. Results from the proposed research will shed new light on the basic biology of HIV-relevant immune function at the vaginal mucosa, to further understand the environment for sexual transmission of HIV, which the NIH has identified as critical to understanding HIV infection rates in women. Defining effects of PVI on immune defenses could have a major impact on the development of novel methods for the prevention or treatment of HIV. For example, identifying aspects of vaginal immunity that are dysregulated by sexual activity could inform the rational design of more effective microbicides.